1. Field of the Invention
The present invention relates to a radioactive-ray image tube for converting a radioactive-ray image into a visible image or an electric image signal, and relates to a manufacturing method thereof. More particularly, the present invention relate to a radioactive-ray image tube capable of preventing the scattering of radioactive rays, particularly, at an input part to improve efficiency in use of radioactive rays, achieving high contrast and high resolution and obtaining a high quality photofluorographic output image as well as a manufacturing method thereof.
Radioactive rays for input part excitation to which the present invention is applicable are ones in a broader sense, including, for example, X rays, .alpha. rays, .beta. rays, .gamma. rays, neutron rays, electron rays and highly charged particle rays.
2. Description of the Related Art
Description will be given to a conventional radioactive-ray image tube, taking a typical X ray image intensifier tube using X rays as radioactive ray as an example. The X ray image tube is installed, as main equipment for examining the internal configuration of a human body or a structure, into an X ray diagnostic apparatus or a nondestructive testing apparatus. The X ray image tube is used to convert the radioactive-ray image of a radioactive-ray transmittance system for examining the transmittance distribution of X rays penetrating through a human body or a structure, into a visible image or an electric image signal.
FIG. 18 is a cross-sectional view showing the schematic structure of a conventional X ray image intensifier tube 1. The X ray image tube 1 comprises a vacuum vessel 2, an X-ray input window 3 formed on one side surface of the vacuum vessel 2 and comprising of Al material curved into a convex shape externally, an input substrate 4 arranged inside the input window 3 to leave a predetermined distance from the window 3, an input screen 5 comprising an input phosphor layer such as CsI layer and a photocathode layer formed on the inner surface of the input substrate 4, an output window 6 formed on the other side surface of the vacuum vessel 2 so as to oppose the input screen 5 and an output screen 7, such as a phosphor layer for observation, formed on the inner surface of the output window 6. Between the input window 3 and the output window 6 are coaxially provided with focusing electrodes 8 and anode 9 in appropriate numbers for the formation of an electrostatic lens system.
An aluminum alloy plate such as A6061P-O material specified by JIS (Japanese Industrial Standard) generally having a thickness of about 0.5 to 3.2 mm, a titanium plate having a thickness of about 0.2 to 0.4 mm or a stainless steel plate have been used as the material of the input window 3. They are selected because it is necessary to have good X ray transmittance characteristics and for the vacuum vessel 2 to sufficiently mechanically withstand external pressure such as atmospheric pressure.
In addition, the input window 3 tends to be easily deformed internally by the external pressure before and after evacuation. Due to this, if the input screen 5 is formed directly inside the input window 3, an output image tends to be distorted. To avoid the influence of the deformation of the input window 3, the input screen 5 is formed on the input substrate 4 which has been separately formed from the input window 3 and arranged to have a distance of, for example, about 10 to 15 mm, from the input window 3.
The above input substrate 4 comprises of a soft pure aluminum material which can be easily smoothed to enhance the adhesion strength (bonding strength) of the input screen 5 and to suppress the irregular reflection of emission light on the surface of the input substrate 4.
In the X ray image tube 1, X ray transmitted through the input window 3 and input substrate 4 is converted into phosphor image by the input screen 5 and further into corresponding photoelectric images by photocathode layer. The photo electrons are accelerated and converged by the electrostatic lens system comprising of the focusing electrodes 8 and anode 9, then the electrons are collided with the output screen 7, thereby obtaining an optical image or electrical image signals.
In the conventional X ray image tube stated above, however, the input substrate is made of soft pure aluminum material. Due to this, to accurately hold the input screen such as a phosphor layer at a predetermined position while maintaining high structural strength, it is necessary to form the input substrate out of a considerably thick material. As a result, the absorption of X rays at the input substrate increase and X rays scatter more greatly, thereby disadvantageously lowering the resolution of the X ray image tube.
Besides, in the conventional X ray image tube, the X ray input part is formed of a double structure comprising of an input window and an input substrate arranged to have a predetermined distance from the input window. Due to this structure, the following disadvantages occur. The absorption and scattering of incident X rays increase at the input part. Efficiency in use of X rays decreases. The brightness, contrast characteristics and resolution of a finally obtained output image greatly deteriorate.
Moreover, since the conventional X ray image tube has a structure in which the input window and the input substrate are separately fabricated and assembled, complex steps are required to manufacture and assemble the X ray image tube and X ray image tube production costs thereby increase.
Meanwhile, to prevent X rays from scattering at the input part, an X ray image tube having an input screen directly formed on the inner surface of the input window has been manufactured. However, this structure has a disadvantage in that plane quality on the inner surface of the input window tends to be non-uniform and coarse, the input phosphor layer such as a CsI deposited film tends to be non-uniform and influenced by distortion, resulting in the aggravated resolution of an output image.